Add Yarn repo

[Daniel15]

Adds repo for Yarn, a new package manager for JavaScript.

Closes #321

[robfrawley]

Will this be merged at any point?

[Daniel15]

I don't know whether it's actually needed - Apparently the repo works even without this. ¯\_(ツ)_/¯

[robfrawley]

Without this pull request, it doesn't work in container-based (non-sudo) builds, does it? If so, how?

[Daniel15]

Someone at work mentioned that this works, however I have not tested it:

addons:
  apt:
    sources:
    - sourceline: 'deb https://dl.yarnpkg.com/debian/ stable main'
      key_url: 'https://dl.yarnpkg.com/debian/pubkey.gpg'

It's pretty weird that Travis' container infra uses this whitelist, given other container-based CI systems (like CircleCI) don't need anything like this. 😕

[robfrawley]

Just wanted to follow-up and mention that your same code works flawlessly; thanks! As for the oddity of both the sudo restrictions and the apt whitelist: I strongly agree.